The present invention relates generally to electronic ballasts configured for parallel discharge lamp operation. More particularly, the present invention relates to a program start ballast having a current-fed, parallel resonant inverter topology.
Referring to FIG. 1, an example is shown of a conventional electronic ballast topology with a current-fed, parallel resonant main circuit 100 for powering a load circuit 101 having multiple lamps coupled in parallel.
The input voltage V_bus may typically be provided from a power factor correction (PFC) section output. Coupled in series between the input voltage V_bus and ground are a pair of relatively large electrolytic capacitors C1, C2 having a substantially equal value. An inverter circuit formed of serially connected switching elements Q1, Q2 is coupled in parallel with the electrolytic capacitors C1, C2, with switching element Q1 coupled in parallel with capacitor C1 and switching element Q2 coupled in parallel with capacitor C2. Free-wheeling diodes D1, D2 are coupled across the switching elements Q1, Q2, respectively.
The primary winding of a choke inductor L_choke_p is coupled between the collector of switching element Q1 and the input voltage V_bus, and the secondary winding of the choke inductor L_choke_s is coupled between the emitter of switching element Q2 and ground. A third capacitor C3 is further coupled in parallel with the inverter and opposite serially connected capacitors C1, C2.
A main resonant tank is coupled between the inverter output and a node between the capacitors C1, C2. The resonant tank is formed of a capacitor C_res in parallel with the primary winding T_res_p of a resonant transformer T_res. Lamps La1, La2 are further coupled across the secondary winding T_res_s of the resonant transformer T_res through capacitors C5, C6, respectively. A parallel resonant tank circuit may therefore be generally described with respect to each lamp coupled to the inverter circuit as including the resonant transformer T_res having a magnetizing inductance in shunt with resonant capacitor C_res and load-coupled capacitors C5, C6 . . . Cn. Prior to ignition of the lamps La1, La2, the input voltage V_bus charges a capacitor C4 through resistor network R3, R4. When the voltage on capacitor C4 reaches a threshold voltage the switching element Q2 may be turned on. In the example shown, the threshold voltage is embodied in the breakdown voltage of a diac 102, wherein the diac breaks down and substantially forms a short circuit such that the charge from capacitor C4 turns on switching element Q2. After the switching element Q2 turns on, the inverter starts to resonate and the secondary winding T_res_s of resonant transformer T_res, along with transformer windings T_res_base_1 and T_res_base_2 providing additional positive feedback via circuit components R2, D3, R1, D4, and driving switching elements Q1, Q2 in a self-oscillating fashion as the inverter reaches steady state.
The current-fed, parallel resonant inverter topology typically is used for instant start electronic ballasts. A particular advantage of this topology is that multiple lamps may be driven in parallel, which means that if one lamp fails other lamps may nevertheless continue to operate. However, instant start ballasts typically have substantially shorter lamp lives than program start ballasts, also referred to as programmed-start, soft-start, rapid-start or preheat ballasts. It would therefore be desirable to combine the advantageous features of the current fed parallel resonant inverter topology and program start ballasts to provide filament preheating for a number of lamps coupled in parallel.
As filament heating may result in a substantial amount of wasted power during steady-state operation of the lamps, it would be even further desirable to remove the filament heating source after the filaments have been properly heated, and during steady-state operation of the lamp.
As a voltage being provided across the lamps during filament preheat operation in many cases is known in the art to produce a small lamp current known as glow current, which causes filament erosion and substantially reduces lamp life, it would be even further desirable to provide little or no voltage across the lamps during the filament preheat period.